As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high power stability, is very high.
Furthermore, secondary batteries may be classified based on the construction of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be constructed in a jelly-roll (winding) type structure in which long-sheet type cathodes and long-sheet type anodes are wound while separators are disposed respectively between the cathodes and the anodes or in a stack type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked while separators are disposed respectively between the cathodes and the anodes.
However, the conventional electrode assemblies have several problems.
First, the jelly-roll type electrode assembly is manufactured by densely winding the long-sheet type cathodes and the long-sheet type anodes with the result that the jelly-roll type electrode assembly is circular or elliptical in section. Consequently, stress, caused by the expansion and contraction of the electrodes during the charge and discharge of a battery, accumulates in the electrode assembly, and, when the stress accumulation exceeds a specific limit, the electrode assembly may be deformed. The deformation of the electrode assembly results in the nonuniform gap between the electrodes. As a result, the performance of the battery is abruptly deteriorated, and the safety of the battery is not secured due to an internal short circuit of the battery. Furthermore, it is difficult to rapidly wind the long-sheet type cathodes and the long-sheet type anodes while maintaining uniformly the gap between the cathodes and anodes, with the result that the productivity is lowered.
Secondly, the stack type electrode assembly is manufactured by sequentially stacking the plurality of unit cathodes and the plurality of unit anodes. As a result, it is additionally necessary to provide a process for transferring electrode plates, which are used to manufacture the unit cathodes and the unit anodes. Furthermore, a great deal of time and effort are required to perform the sequential stacking process, with the result that the productivity is lowered.
In order to solve the above-mentioned problems, there has been developed a stack/folding type electrode assembly, which is a combination of the jelly-roll type electrode assembly and the stack type electrode assembly. The stack/folding type electrode assembly is constructed in a structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked, while separators are disposed respectively between the cathodes and the anodes, to constitute a bi-cell or a full-cell, and then a plurality of bi-cells or a plurality of full-cells are wound while the bi-cells or the full cells are located on a long separator sheet. The details of the stack/folding type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-0082058, No. 2001-0082059, and No. 2001-0082060, which have been filed in the name of the applicant of the present patent application.
FIGS. 1 and 2 typically illustrate an exemplary structure of such a stack/folding type electrode assembly and a process for manufacturing the stack/folding type electrode assembly, respectively.
Referring to these drawings, a plurality of full cells 10, 11, 12, 13, 14 . . . , as unit cells, constructed in a structure in which a cathode, a separator, and an anode are sequentially arranged are overlapped such that a separator sheet 20 is disposed between the respective full cells. The separator sheet 20 has unit lengths sufficient to surround the respective full cells. The separator sheet 20 is bent inward every unit length to successively surround the respective full cells from the central full cell 10 to the outermost full cell 14 such that the separator sheet 20 is disposed between the overlapped full cells. The outer end of the separator sheet 20 is finished by thermal welding or an adhesive tape 25.
The stack/folding type electrode assembly is manufactured, for example, by arranging the full cells 10, 11, 12, 13, 14 . . . on the long separator sheet 20 and sequentially winding the full cells 10, 11, 12, 13, 14 . . . from one end 21 of the separator sheet 20.
When carefully observing the array combination of the full cells as the unit cells, the first full cell 10 and the second full cell 11 are spaced from each other by a distance equivalent to the width corresponding to at least one full cell. Consequently, during the winding process, the outside of the first full cell 10 is completely surrounded by the separator sheet 20, and then a bottom electrode of the first full cell 10 faces a top electrode of the second full cell 11.
During the sequential stacking of the second full cell and the following full cells 11, 12, 13, 14 . . . through the winding, the surrounding length of the separator sheet 20 increases, and therefore, the full cells are arranged such that the distance between the full cells gradually increases in the winding direction.
Also, during the winding of the full cells, it is required for cathodes of the full cells to face anodes of the corresponding full cells at the interfaces between the respective full cells. Consequently, the first full cell 10 and the second full cell 11 are full cells of which the top electrode is a cathode, the third full cell 12 is a full cell of which the top electrode is an anode, the fourth full cell 13 is a full cell of which the top electrode is a cathode, and the fifth full cell 14 is a full cell of which the top electrode is an anode. That is, except the first full cell 10, the full cells of which the top electrode is a cathode and the full cells of which the top electrode is an anode are alternately arranged.
Consequently, the stack/folding type electrode assembly considerably makes up for the defects of the jelly-roll type electrode assembly and the stack type electrode assembly. In order to stack the bi-cells or the full cells, which are the unit cells, such that the bi-cells or the full cells have opposite electrodes at the interfaces between the unit cells, however, it is required to sort the bi-cells or the full cells based on the type of the bi-cells or the full cells and load the bi-cells or the full cells on the separator sheet according to a predetermined rule, with the result that the manufacturing process is complicated and troublesome, and therefore, the productivity is lowered. Furthermore, it is very complicated and troublesome to sort the unit cells based on the type of the unit cells. Consequently, when even any one of the unit cells is omitted or wrongly located due to various causes, such as carelessness or errors, during loading the unit cells on the separator sheet, electrodes having the same polarity face each other at the interfaces between the unit cells, with the result that the performance of the battery is deteriorated.
In conclusion, the stack/folding type electrode assembly is preferred in the aspect of the operational performance and safety of the battery. However, the stack/folding type electrode assembly is disadvantageous as described above in the aspect of the productivity of the battery. Consequently, there is a high necessity for an electrode assembly that is capable of providing higher productivity and operational performance of the battery while making up for the above-mentioned defects.
In particular, a large number of battery cells (unit cells) are necessary to manufacture a large-sized battery module, used for middle- or large-sized devices, such as electric vehicles and hybrid electric vehicles, in which much interest has been taken in recent years. Also, the large-sized battery module requires a long-term life span property. Consequently, there is a high necessity for an electrode assembly having a specific structure to solve all the above-mentioned problems.